High definition television (HDTV) signals typically are not compatible with current broadcast standards such as NTSC. A preferred signal coding standard suitable for use with the HDTV compatible signals is MPEG-2 (Motion Picture Experts Group, "Information Technology--Coding of Moving Pictures and Associated Audio Information: Video," ISO/IEC 13818-2, May 15, 1996) as adopted by the Grand Alliance for use in the United States. This well-known standard provides formatting guidelines for coding and compressing video, audio and auxiliary data.
Formatting television signals with the MPEG-2 standard required both television broadcast transmission and receiving system architectures to be redesigned. Designers introduced compression systems which reduce the amount of broadcast data. HDTV receivers decode and decompress the received coded HDTV data and reconstruct a full resolution signal for display. Losses in the signal are minimal because of the sophistication of the coding/decoding and reconstruction techniques employed, and expected losses typically occur where the human eye is perceptually limited, for example, in fine detail of diagonal motion.
Whereas much research and development has occurred in all aspects of transmission and reception of HDTV, designers are focusing on HDTV receivers because of competition to produce a consumer product with the best features at the lowest cost. The first consumer HDTV receivers may include fewer features than current standard definition television receivers, but will offer new features such as a 16:9 display. Manufacturers are now designing consumer receivers which not only include features available today in standard definition receivers, such as PIP, but also features which support other applications, such as interactive television and Internet access. Manufacturers are challenged to design system architectures which incorporate into the HDTV receiver new services and features not currently available in a TV receiver, or which are only available via a set top box or some other interface.
HDTV receivers will be able to display graphics, for example a spread sheet program, as well as traditional programming. Displaying graphics may also occur if the viewer is using the HDTV receiver to access information on the Internet, for example. The viewer may wish to monitor traditional high definition (HD) programming or another HD video signal at the same time the graphics are being displayed. This is known as picture-in-graphics, or PIG. Or the viewer may wish to view two programs, at least one of which is HD, using the picture-in-picture (PIP) or picture-on-picture (POP) feature. An HDTV signal which is transmitted according to the MPEG2 standard presents a different set of design problems to overcome because the video data is formatted into blocks and macroblocks instead of lines.
Presenting a second, smaller image in a larger image by PIP, POP or PIG requires the smaller image to be filtered and subsampled to attain the desired size for display. This is easily done in the horizontal direction. Vertical decimation of a video signal, however, has always been a challenge to designers when dealing with an interlaced video signal. Vertical decimation of an interlaced signal produces noticeable artifacts because the phase of the new lines must be calculated. To get the correct phase, either the preceding or following field must be available, requiring storage of the needed field in memory. This increases memory and bandwidth requirements. To reduce aliasing, the signal must be band limit filtered, but without all of the lines available to the filter, the resulting spectrum is incorrect.
These traditional problems are compounded by the complexity of pixel blocks and macroblocks of the MPEG2 format. Traditional line memories are not sufficient to accommodate an input data stream which is not in line by line order. Known HDTV receivers reformat the data to raster scan format before converting the full resolution signal to the PIP, POP or PIG signal to be displayed.